The Global Positioning System (GPS) has been used for more than a decade for the accurate determination of position on the earth's surface, as well as navigation. The system consists of approximately thirty satellites, managed by the US Department of Defense, orbiting at an altitude of 20 200 kilometres, as well as thousands of stationary ground-based and mobile receivers. It has become apparent from numerous studies that the delay of GPS signals in the atmosphere can also be used to study the amosphere, particularly to determine the precipitable water vapour (PWV) content of the troposphere and the total electron content (TEC) of the ionosphere. This dissertation gives an overview of the mechanisms that contribute to the delay of radio signals between satellites and receivers. The dissertation then focuses on software developed at the Hartebeesthoek Radio Astronomy Observatory's (HartRAO's) Space Geodesy Programme to estimate tropospheric delays (from which PWV is calculated) in near real-time. In addition an application of this technique, namely the improvement of tropospheric delay models used to process satellite laser ranging (SLR) data, is investigated. The dissertation concludes with a discussion of opportunities for future work. / Thesis (M.Sc. (Physics))--North-West University, Potchefstroom Campus, 2004.

Diss. (sammanfattning) Alnarp : Sveriges lantbruksuniv. / Härtill 5 uppsatser.

Die Signale des Global Positioning Systems (GPS) ermöglichen nicht nur die präzise Positionsbestimmung an jedem Ort der Erde, sondern enthalten auch Informationen über den Wasserdampfgehalt der Atmosphäre. Aus diesen Informationen kann mit Hilfe tomographischer Verfahren die dreidimensionale Wasserdampfverteilung in der Troposphäre rekonstruiert werden. Dabei ist es von besonderem Interesse, wie zuverlässig die Wasserdampfverteilung alleine aus den Daten bodengestützter GPS-Netzwerke, d. h. ohne die Einbeziehung von Low Earth Orbitern (LEOs) oder von Wettersatelliten, bestimmt werden kann. Zu diesem Zweck werden GPS-Daten für unterschiedliche GPS-Netzwerke simuliert und auf ihre Eignung für eine tomographische Rekonstruktion untersucht. Besonders der Zusammenhang zwischen der Anzahl der Satelliten, der Stationsdichte und der in der tomographischen Rekonstruktion erreichbaren Auflösung ist dabei von großer Bedeutung. Aus diesem Grund erfolgt zunächst eine geometrische Bewertung der GPS-Eingangsdaten. Die Rekonstruktion eines 3D-Feldes und die dabei auftretenden Abweichungen zwischen dem originalen und dem rekonstruierten Feld werden abschließend an einem Beispiel dargestellt. / The signals of the global positioning system (GPS) can not only be used to obtain the precise position all over the world but carry also information about the water vapour distribution in the atmosphere. This 3D distribution can be reconstructed from the GPS data by means of tomographic techniques. Of special interest is the quality of the reconstructed 3D distribution using data from GPS ground stations alone, without regarding low earth orbiters (LEOs) and weather satellites. Therefore, GPS data have been simulated for different model networks. A subsequent data validation shows the correlation between the number of GPS satellites, the density of ground stations and the spatial resolution of the water vapour field which might be obtained from a tomographic reconstruction. Some selected data sets have been used to reconstruct a given 3D distribution and to analyse variations between the initial model field and the results obtained with different tomographic techniques.

Les matériaux basés sur le système Fe-Ni-Cr utilisés à haute température doivent présenter une bonne résistance à l'oxydation, généralement obtenu par la croissance lente d’une couche d’oxyde de chrome à la surface de ces alliages. Pour prédire la formation d'une couche d’oxyde protectrice la perméabilité de l'oxygène dans l’alliage doit être connue, la perméabilité étant définie comme le produit de la solubilité et du coefficient de diffusion de l’oxygène. L'objectif de nos travaux est de mesurer la perméabilité, la solubilité et la diffusivité de l'oxygène dans des alliages Fe-Ni pour des températures supérieures à 1000°C. Afin d'obtenir les meilleurs résultats,la formation d'une couche externe d'oxyde pendant les expériences doit être évitée. Pour cela, la pression partielle d'oxygène a été fixée à la pression d'équilibre Fe/FeO dans toutes les expériences. En outre, afin d'étudier l'effet de la vapeur d'eau sur la perméabilité, la solubilité et la diffusivité de l’oxygène, deux atmosphères ont été utilisées: l’une considérée comme sèche et l’autre comme humide. L'atmosphère sèche a été obtenue en utilisant la technique du « pack de Rhines »: les échantillons sont oxydés dans des capsules en quartz sous vide qui contiennent un mélange de poudre de fer et de wüstite. L'atmosphère humide a quant à elle été obtenue en utilisant des mélanges gazeux H2/H2O avec un ratio approprié de vapeur d'eau et d’hydrogène afin de fixer la pression partielle d’oxygène à la pression d’équilibre Fe/FeO. Les mesures de solubilité réalisées dans ce travail ont montré que celle-ci atteint son maximum dans le fer pur et diminue avec l’ajout de nickel. Cependant, la dépendance de la solubilité avec la composition de l’alliage Fe-Ni n'est pas idéale et ne peut être prédite à partir de modèles simples. De plus, les résultats obtenus sous atmosphère humide suggèrent que la présence de vapeur d'eau dans l'atmosphère augmente la solubilité de l'oxygène d’un facteur 2 dans les alliages avec une concentration en nickel inférieure ou égale à 60 at.% pour des températures proches de 1000°C,tandis qu'à 1150°C, la solubilité de l'oxygène est indépendante de l'environnement. La perméabilité de l'oxygène a été déterminée en mesurant la cinétique d'oxydation interne d'alliages Fe-Ni-Cr. Les résultats ont montré que la perméabilité de l'oxygène présente les mêmes variations avec la composition de l’alliage que la solubilité de l'oxygène, indépendamment de l'atmosphère.De plus, aucun effet significatif de la vapeur d'eau sur les valeurs de perméabilité de l'oxygène n'a été observé. Le coefficient de diffusion de l'oxygène a également été déterminé en utilisant les résultats précédents, c’est à dire la perméabilité et la solubilité de l’oxygène mesurées dans notre étude. Pour une température supérieure à 1000°C, la variation du coefficient de diffusion de l'oxygène avec la composition de l'alliage est similaire dans tous les environnements testés et un maximum est observé pour les alliages avec une teneur en nickel de 40 at.%. Cependant, la présence de vapeur d'eau dans l'atmosphère diminue les valeurs du coefficient de diffusion de l'oxygène, par un facteur 2-3 à 1000°C, pour les alliages avec une concentration en nickel inférieure ou égal à 60 at.%. De plus, il a été trouvé que la différence entre les coefficients de diffusion mesurés dans l’atmosphère sèche et humide augmente à mesure que la température diminue. En conclusion, il a été constaté que la vapeur d'eau n'a aucun effet sur la manière dont la perméabilité, la solubilité et la diffusivité de l'oxygène varient avec la composition des alliages Fe-Ni. Cependant, la présence de vapeur d'eau dans l'environnement semble augmenter la solubilité de l'oxygène et diminuer sa diffusivité dans les alliages riches en fer. De plus, l’effet de la vapeur d’eau apparaît plus important aux plus basses températures étudiées. / It is a worldwide priority to reduce emissions of greenhouse gases such as CO2. One solution for reducing these emissions is to improve the efficiency of energy production units by increasing their operating temperature. However, in order to increase operating temperature, new austenitic materials based on the Fe-Ni-Cr system have to be designed. In addition, these materials need to exhibit good protection against high temperature oxidation, which is achieved by the formation of a slow growing chromium oxide or alumina scale on the metal. However, to predict the formation of a protective scale, knowledge of the oxygen permeability, the product of oxygen solubility and diffusivity, in the base alloy is required. The objective of this study is to measure the permeability, solubility and diffusivity of oxygen in Fe- Ni alloys at temperatures above 1,000°C. In order to obtain the best results, the formation of an external oxide layer during the experiment has to be avoided. To achieve this, the oxygen partial pressure was fixed at the Fe/FeO equilibrium pressure in all experiments. In addition, two types of atmospheres were used: one dry and one wet, in order to investigate the effect of water vapour on oxygen permeability, solubility and diffusivity. The dry atmosphere was achieved using the Rhines Pack technique. The samples were oxidised in vacuum-sealed quartz capsules, which contained a mixture of powdered iron and wüstite. The humid atmosphere was obtained by using H2/H2O gas mixtures with the appropriate water vapour to hydrogen ratio to fix oxygen partial pressure at the Fe/FeO equilibrium. The maximum oxygen solubility was found in pure iron, and decreased continuously with nickel additions to the alloy. The dependence of solubility on alloy composition is non-ideal, and cannot be predicted from simple models. Moreover, the presence of water vapour in the atmosphere seems to increase the solubility by a factor of 2 in alloys with nickel content lower than 80 at.% at temperatures near 1,000°C. However, at 1,150°C the solubility of the oxygen is independent of the environment. The oxygen permeability was determined by measuring the internal oxidation kinetics of Fe-Ni-Cr alloy. These kinetics were evaluated by measuring the internal oxidation zone depth by optical microscopy, or by continuous and discontinuous thermogravimetry. Results showed that the oxygen permeability exhibits the same variation with alloy composition as the oxygen solubility, independent of the atmosphere. In particular, no significant effect of water vapour on oxygen permeability values was observed. In the present study, the oxygen diffusion coefficient was also determined using permeability, in addition to the independent measurement of the oxygen solubility carried out in the present study. For temperature above 1,000°C, the variation of oxygen diffusion coefficient with the alloy composition is similar in all environments tested, and a maximum is observed for alloys with a nickel content of 40 at.%. However, for a given nickel content up to 60 at.%, the presence of water vapour in the atmosphere decreases the value of the oxygen diffusion coefficient by a factor of 2-3 at 1,000°C. In addition, this difference between diffusion coefficients measured in a dry and wet atmosphere increases as the temperature decreases. Overall, it was found that the water vapour has no effect on the way in which oxygen permeability, solubility and diffusivity vary with the alloy composition. However, the presence of water vapour in the environment appears to increase the oxygen solubility and decrease the oxygen diffusivity in iron-rich alloys, the effect being more significant at low temperatures. These results suggest further research into interactions between O, H and metal vacancies, particularly for temperature around 1,000°C and below, as the latter defect is thought to change the diffusion and solubility properties of interstitial species.

his thesis deals with the problems of cavitation flow in a Venturi tube. It is made for a multi-phase (water-vapour) flow calculation of two geometry tubes. Results of numerical calculations for the flow geometry (dimensions) are compared with performed experiment. Numerical flow calculation was performed in the Fluent 2.3.26.

Radio signals from Global Navigation Satellite Systems (GNSS) satellites suffer delay as they propagate through the atmosphere (neutral and non-neutral) and this delay is partially driven by the water vapour content in the atmosphere. The delay component due to the non-neutral atmosphere (ionosphere) is removed through the use of dual frequency GNSS receivers. The main tropospheric parameter is the zenith tropospheric (or total) delay (ZTD), which is a widely accepted parameter with which to express the total delay in the signal from all satellites due to the neutral atmosphere. The ZTD is a measure of the integrated tropospheric condition over a GNSS receiver station. Accordingly, the integrated water vapour or precipitable water vapour (PWV) can be obtained from a portion of the ZTD, if the atmospheric pressure and temperature at the station are known through a concept often referred to as GNSS meteorology. A number of GNSS receivers have been deployed for mapping and geodetic services in Nigeria under the African reference frame initiative, but unfortunately most of these receivers do not have co-located meteorological sensors for pressure and temperature measurements. The prospect of incorporating GNSS meteorology into weather monitoring and climate analysis in Nigeria was investigated and is reported in this thesis. During the first task of this research, the technical basis for ground-based GNSS meteorology was reviewed and the potentials and challenges of the approach to meteorological activities in Africa (including Nigeria) were identified. Thereafter an in-depth analysis of the spatial and temporal variability of ZTD over Nigeria for the period of 2010-2014 was conducted; results revealed weak spatial dependence among the stations. Tidal oscillations (of the diurnal and semidiurnal components) were observed at the GNSS stations of which the diurnal ZTD cycles exhibited significant seasonal dependence, affirming the prospective relevance of ground-based GNSS data to atmospheric studies. Also in this research, the accuracy and suitability of using reanalysis datasets (ERA-Interim and NCEP/NCAR) and a GPT2 neutral model in retrieving PWV from GNSS observations over Nigeria were investigated; results showed that PWV can be retrieved to within a precision of about 1 mm, provided GNSS-derived ZTD is of high precision. A fundamental issue for GNSS meteorology in the West African region was yet again addressed in this research; this is the development of a weighted tropospheric mean temperature model for use in current and future GNSS meteorology activities in the region. A multitechnique comparison of PWV estimates showed good agreement between GNSS estimates and other techniques (i.e. the atmospheric infrared sounder, and ERAInterim reanalysis). This result is suggestive of the potential of assimilating GNSS atmospheric products into reanalysis and climate models. Diurnal and seasonal variability of GNSS PWV estimates exhibits strong correlation with weather events that influence the region (i.e. solar activity and rainfall events); this further demonstrated the immense contribution of the approach to efficient weather forecasting and climate monitoring for Nigeria. / Thesis (PhD)--University of Pretoria, 2017. / Geography, Geoinformatics and Meteorology / PhD / Unrestricted

Since 2008 a group of scientists of the Leipzig Institute of Meteorology (LIM) and the German Research Centre for Geosciences Potsdam (GFZ) develops a method to derive water vapour profiles out of continuously available GNSS data (Global Navigation Satellite System). The aim of this project - supported by the Deutsche Forschungsgemeinschaft (DFG) - was to develop a processing system with related scientific algorithms, which uses data of regional GNSS ground networks to derive 3D water vapour distributions above these stations. This systems use the line of sight water vapour information from each ground station to every GNSS satellite in view (slants) as basis of a 3D tomographic reconstruction. At this time these reconstructions are based on GNSS data of about 330 German or near Germany located groundstations. This leads to a horizontal resolution of the reconstructed 3D water vapour field up to 40km and a vertical resolution of about 0.5km from the upper part of troposphere down to the atmospheric boundary layer (1km height). / Seit 2008 befasst sich eine Arbeitsgruppe von Wissenschaftlern am LIM und dem GFZ in Potsdam im Rahmen eines DFG-geförderten Projektes mit der Ableitung von dreidimensionalen Wasserdampfverteilungen in der Atmosphäre aus Beobachtungsdaten regionaler GNSS-Bodennetze (GlobaleNavigationsSatellitenSysteme). Die Wasserdampfverteilungen können aus der atmosphärischen Information entlang der Sichtlinien zwischen den Bodenstationen und den sichtbaren GNSS-Satelliten (sogenannte Slants) berechnet werden. Diese zahlreichen Sichtlinien ermöglichen eine tomographische Verarbeitung der Daten. Der entwickelte tomographische Algorithmus nutzt derzeit bis zu 330 deutsche und nahe Deutschland gelegene GNSS-Stationen, was eine horizontale Auflösung der resultierenden 3D-Felder von 40km und einer vertikalen Auflösung von 0,5km bis hinab zur atmosphärischen Grenzschicht (bis 1 km über dem Boden) ermöglicht

La technologie de l’électrolyse de la vapeur d’eau à haute température (EVHT) est unesolution alternative à la production d’hydrogène. Le principe est inversé à celui d’une pile àcombustible de type SOFC : on utilise la vapeur d’eau et de l’électricité afin de produire del’hydrogène. Une difficulté technique majeure repose sur la mise au point d’interconnecteursfonctionnant efficacement sur le long terme. Sur le plan électrique, l’interconnecteur doitprésenter une valeur de résistance de contact aux électrodes la plus faible possible, car elleaffecte directement le rendement de conversion électrochimique (eau en hydrogène) et peutpénaliser le procédé. Il ne doit donc pas présenter une cinétique d’oxydation élevée ni formerdes oxydes isolants électriquement. Sur le plan chimique, l’interconnecteur doit être résistantà l’oxydation sous atmosphère riche en oxygène côté anode et riche en vapeur d’eau côtécathode. De plus, le problème de la volatilisation des oxydes de chrome, qui peuvent diffuseret empoisonner les électrodes, déterminant ainsi une réduction de l’activité électrochimique etdes performances du « stack » sur des longues durées de fonctionnement, doit être réduit. Latempérature de fonctionnement comprise entre 700 et 900°C permet l’utilisationd’interconnecteurs métalliques, qui présentent l’avantage d’une mise en oeuvre plus facile etd’un coût plus faible par rapport aux interconnecteurs céramiques.Dans cette étude, deux matériaux ont été testés en tant qu’interconnecteurs pour lessystèmes EVHT : un acier ferritique chromino-formeur K41X et un alliage Fe-Ni-Co necontenant pas de chrome. Le comportement envers la corrosion à haute température et laconductivité électrique des deux alliages ont été évalués à 800°C sous un mélange 95%O2-5%H2O, pour le côté anodique, et 10%H2-90%H2O, pour le côté cathodique. Pour l’alliageK41X, l’effet de l’état initial de la surface des échantillons sur la nature des oxydes formés àhaute température sous mélange H2-H2O a été pris en compte, à travers une comparaison desalliages bruts de laminage avec des surfaces polies miroir. L’effet d’une pré-oxydation decourte durée à 800°C sur le comportement à haute température de l’alliage K41X brut deréception sous atmosphère H2-H2O a également été évalué. Mais, le travail le plus original decette étude a consisté à effectuer des essais de marquage à l’or et des marquages isotopiquessous mélange H216O-H218O, H2-D2O et D2-H2O. Ces tests ont permis d’étudier lesmécanismes responsables de la croissance de la couche de corrosion de l’alliage K41X brut deréception et poli miroir à 800°C sous atmosphère H2-H2O et d’évaluer le rôle de la vapeurd’eau et de l’hydrogène dans le mécanisme d’oxydation / The high temperature water vapour electrolysis offers a promising method for highlyefficient hydrogen production. It works as an inverse solid oxide fuel cell, using water vapourand electricity in order to produce hydrogen. A major technical difficulty related to hightemperature water vapour electrolysis (HTVE) is the development of interconnects workingefficiently on a long period. From the electrical point of view, the interconnect must have alow contact resistance with the electrodes. Indeed, it directly affects the electrochemicalconversion efficiency (water into hydrogen) and it can penalize the process. The interconnectmust present a slow oxidation kinetics and form as less as possible electrical insulatingoxides. From the chemical point of view, the interconnect has to be resistant against oxidationin an oxygen rich atmosphere (anode side) and water vapour rich atmosphere (cathode side).Moreover, the problem of the volatility of chromium oxide species, which might migrate andpoison the electrodes, leading to a decrease in their electrochemical activity and degradationof stack performance, over long-term operation, needs to be reduced. The operatingtemperature between 700°C and 900°C allows the use of metallic interconnects, which havehigher electrical and thermal conductivities, easier shaping and lower cost, with respect to theceramic materials.In this study, two materials were tested as interconnects for the HTVE systems: a ferriticchromia-forming alloy, the K41X, and a Fe-Ni-Co alloy, which does not contain chromium.High temperature corrosion behaviour and electrical conductivity were tested in both anode(95%O2-5%H2O) and cathode (10%H2-90%H2O) atmospheres at 800°C. Moreover, for theK41X alloy, the effect of the initial surface state of the samples on the chemical nature of theoxides formed at 800°C in H2-H2O atmosphere was evaluated, by comparing as received andmirror polished surfaces. The effect of a short-term air preoxidation at 800°C on the hightemperature behaviour of the K41X as received sample in H2-H2O atmosphere was tested.The most original part of this study consisted in the investigation of the oxidation mechanismsof both as received and mirror polished K41X samples at 800°C in H2-H2O atmosphere bymeans of marking experiments using Au and isotopes (H216O-H218O mixture). Moreover,marking tests using H2-D2O and D2-H2O were carried out, in order to further investigate therole of hydrogen and water vapour in the oxidation mechanism

In thesis is dealt with problematic of how the value of air permeability may be affected by properties for water vapour diffusion with regard to OSB material There were chosen two main brands of OSB at Czech trade market Two specimens of OSB with different thickness were chosen for each brand. As a first the specimens were tested for air tightness to work out a value of air permeability for each specimen. Afterwords, the testing was conducted for gaining properties of water vapour diffusion on each specimen. As a final assessment was analysed the impact of properties of water vapour diffusion on the air tightness for each specimen. The general meaning of gaining higher air tightness with higher water vapour resistance was proved . It was shown that with raising thickness is usually gotten higher value of water vapour diffusion resistance. However it could not mean getting high air tightness. The crucial parameter is the quality of the material for gaining higher resistance against water vapour diffusion as well as for higher airtightness.

Thesis (MScEng (Mechanical and Mechatronic Engineering))--University of Stellenbosch, 2010. / AFRIKAANSE OPSOMMING:Die navorsing wat hier uiteengesit word ondersoek die hipotese dat kondensasie en die gevolglike verdamping van water wat teenwoordig is in die uitlaatgas van ‘n binnebrandenjin, plaasvind in die gedeeltes van die uitlaatstelsel tussen die uitlaatklep en die katalitiese-omsetter se uitlaat. Daar word verder veronderstel dat hierdie tweefasevloeieffekte die tydafhanklike temperatuurprofiele in die uitlaatstelsel beïnvloed, wat moontlik kan lei tot ‘n vertraging in die tydsduur vir die katalitiese omsetter om temperature van 200-300 °C te bereik, wat nodig is om noemenswaardige omsetting te bewerkstellig. Om hierdie veronderstelling te evalueer is ‘n tydafhanklike, eendimensionele wiskundige model van die termo-vloei gedrag in die uitlaatstelsel gedurende ‘n koue inwerktreeding, insluitende vogtigheidseffekte, opgestel en opgelos deur van ‘n rekenaaralgoritme gebruik te maak. Warmte- en massaoordragsteorie was gebruik om die ongestadigde massa-, energie- en momentumbehoudsvergeleikings te formuleer. Die tweefasige vogeffekte was gemodelleer deur gebruik te maak van die verhouding tussen warmte- en massaoordrag, wat verdamping en heterogeniese kondensasie (die kondensasie van damp teen die pypwand) voorspel as gevolg van die dampdrukgradient tussen die grootmaat damp en die versadigde damp by die oppervlak van die vloeistoffilm. Homogene kondensasie (die kondensasie van vloeistof in die vorm van druppels in die dampstroom) was ook in aanmerking geneem indien die grootmaatgas temperatuur onder die versadigingstemperatuur van die grootmaatdamp gedaal het. ’n Eksperimentele ondersoek was gedoen deur van twee enjins gebruik te maak, ’n 1.6 L Volkswagen Bora en ’n 1.6 L Ford RoCam, in die toetsselle van Cape Advanced Engineering Pty (Ltd). Om die gastemperature so akkuraat moontlik te meet, was spesiale radiasiegeskermde sensore met vinnige reaksietyd ontwerp en installer in die pypseksies van die uitlaatstelsels van beide enjins. Die geskermde sensore het temperature van tot 50 °C hoër as konvensionele termokoppels in dieselfde areas gemeet. Dit is in koers is met resultate wat deur die foutbeperkingsteorie, geassosieer met die meet van temperature in vloeïende gas in uitlaatstelsels, voorspel word. Vergelyking van die numeriese simulasie met die eksperimenteel gemete temperature het aangedui dat in dele van die uitlaatstelsel voor die katalitieseomsetter, die vog min uitwerking het op die termiese gedrag van die stelsel. In hierdie gedeeltes is die konveksie warmte-oordrag dominant. In die katalitieseomsetter was die vogeffekte invloedryk. Die eksperimentele resultate toon ‘n duidelike vroeë toename in die gastemperature, gevolg deur ‘n tydperk van konstante temperature by nagenoeg die versadigingstemperatuur van die grootmaatdamp (verwys na as die temperatuurplato) by die katalitiese-omsetter se kern en uitlaat. Die numeries gesimuleerde gastemperature het ook hierdie gedrag getoon, maar ‘n baie hoë en skerp piek by die begin van die plato het voorgekom. Hierdie piek was nie te sien by die eksperimentele resultate nie en is toegeskryf aan nie-ewewigstoestande in die verdampingsproses, wat aandui dat die tempo van verdamping wat deur die massa-oordragmodel voorspel word te hoog is vir die model en dat dit verfyn moet word. Verdere ondersoek van die invloed van die individuele massa-oordragprosesse het getoon dat die homogene kondensasie die dominante proses is in die vorming van vloeistof in die katalitiese-omsetter. Heterogeniese kondensasie het plaasgevind, maar ‘n kleiner massa vloeistof is produseer. Die maksimum hoeveelheid vloeistof wat voorspel is om in die katalitiese-omsetter te vorm was 12 g/cm wat gelykstaande is aan ‘n film van 0.05.mm dik indien eweredig versprei oor die binneoppervlak van die monoliet. Daar was in die simulasie gevind dat beide verdamping en kondensasie benodig word om die temperatuurplato te simuleer, vanwaar die gevolgtrekking gemaak kan word dat beide prosesse wel plaasvind en dat die eerste stelling in die oorspronklike hipotese wel geldig is. Daar was egter teen die einde van die toetsperiode gevind dat beide temperature wat met en sonder vogeffekte simuleer was, die eksperimentele temperature nagevolg het, wat aandui dat die invloed van vog beperk is tot die vroeë stadiums van die katalitiese-omsetter se opwarmingstydperk. Die tweede gedeelte van die hipotese wat veronderstel dat die voggedrag ‘n vertraging in die tydsduur om omsetting te bewerkstellig veroorsaak, is dus bevind om ongeldig te wees. Die wiskundige model wat opgestel is tydens die ondersoek is weens noodsaaklikheid ‘n vereenvoudigde simulasie van komplekse termo-vloei prosesse. Dit dien as nuttige grondwerk vir verdere in-diepte ondersoeke en afronding van die teorie met betrekking tot voggedrag en die uitwerking daarvan op die tydsafhanklike temperature in ‘n uitlaatstelsel. / ENGLISH ABSTRACT: The research presented here investigates the hypothesis that condensation and subsequent evaporation of water vapour present in the exhaust gas of an internal combustion engine occur in the sections of the exhaust system between the exhaust port and the catalytic converter exit. It is further hypothesised that these two-phase moisture effects influence the transient temperature profiles in the exhaust system, and potentially cause a delay in the time it takes for the catalytic converter to reach temperatures of 200-300 °C, which are required for light-off to occur. In order to evaluate this hypothesis a transient, one-dimensional mathematical model of the thermo-fluid behaviour in the exhaust system during a cold start, including moisture effects, was created and solved by means of a computer algorithm. Heat and mass transfer theory was used to formulate the unsteady conservation equations for mass, energy and momentum. The two phase moisture effects were modelled using the analogy between heat and mass transfer, which predicts evaporation and heterogeneous condensation (the condensation of vapour against the pipe wall) due to a vapour pressure gradient between the bulk vapour and a saturated vapour at the surface of the liquid film. Homogeneous condensation (the condensation of liquid in the form of droplets in the gas stream) was also accounted for if the bulk gas temperature dropped below the bulk vapour saturation temperature. An experimental investigation was performed using two engines, a 1.6.L Volkswagen Bora and a 1.6.L Ford RoCam, in the test cells of Cape Advanced Engineering Pty (Ltd). In order to measure the gas temperatures as accurately as possible specialised radiation shielded sensors with fast time response were designed and installed in the pipe sections of the exhaust systems of both engines. The shielded sensors measured temperatures up 50 °C higher than the conventional thermocouples installed at the same positions, which is in keeping with the results predicted by the theory governing errors associated with temperature measurement in the flowing gas in the exhaust system. Comparison of the numerically simulated and experimentally measured temperatures indicated that in the sections of the exhaust system leading up to the catalytic converter the moisture has little influence on the temperature behaviour of the exhaust system. In these sections the convective heat transfer is dominant. In the catalytic converter the moisture effects were found to be influential. The experimental results clearly show an early rise in the gas temperatures, followed by a period of constant temperature at approximately the saturation temperature of the bulk vapour (referred to as the temperature plateau) at the catalytic converter mid-bed and exit. The numerically simulated gas temperatures also exhibited this plateau, but an initial very high and sharp peak in the simulated gas temperatures occurred at the start of the plateau. This was not seen in the experimental results and is attributed to non-equilibrium in the evaporation process, indicating that the rate of evaporation predicted by the mass transfer model used is too high for this application and that the model needs to be refined. Further investigation of the influence of the individual mass transfer processes indicated that the homogeneous condensation is the dominant process in the formation of liquid in the catalytic converter. Heterogeneous condensation was found to occur, but produced a smaller mass of liquid. The maximum amount of liquid predicted to form in the catalytic converter was 12 g/cm, which translates to a film 0.05 mm thick if evenly distributed over the inner surface of the monolith. In the simulation it was found that both evaporation and condensation are needed in order to simulate the temperature plateau, from which it was concluded that both these processes do occur and the first statement in the original hypothesis is valid. However, by the end of the test period temperatures simulated both with or without the moisture effects closely approached the final temperatures of the experimental investigation, indicating that the influence of the moisture is limited to the early stages of the catalytic converter warm-up. The second part of the hypothesis, postulating that the moisture behaviour caused a delay in the time taken to reach light-off temperature, is therefore concluded to be invalid. The mathematical model constructed in this research is by necessity a simplified solution to complex thermo-fluid processes. It serves as useful groundwork for further elaboration and refinement of the theory related the moisture behaviour and its influence on the transient temperatures in the exhaust system.